Rapid flares from blazars in very high energy ( VHE ) \gamma -rays challenge the common understanding of jets of active galactic nuclei ( AGNs ) .
The same population of ultra-relativistic electrons is often thought to be responsible for both X-ray and VHE emission .
We thus systematically searched for X-ray flares at sub-hour timescales of TeV blazars in the entire Rossi X-ray Timing Explorer archival database .
We found rapid flares from PKS 2005 - 489 and S5 0716 + 714 , and a candidate rapid flare from 1ES 1101 - 232 .
In particular , the characteristic rise timescale of PKS 2005 - 489 is less than half a minute , which , to our knowledge , is the shortest among known AGN flares at any wavelengths .
The timescales of these rapid flares indicate that the size of the central supermassive black hole is not a hard lower limit on the physical size of the emission region of the flare .
PKS 2005 - 489 shows possible hard lags in its flare , which could be attributed to particle acceleration ( injection ) ; its flaring component has the hardest spectrum when it first appears .
For all flares , the flaring components show similar hard spectra with \Gamma = 1.7 - 1.9 , and we estimate the magnetic field strength B \sim 0.1–1.0 G by assuming synchrotron cooling .
These flares could be caused by inhomogeneity of the jets .
Models that can only produce rapid \gamma -ray flares but little synchrotron activity are less favorable .